This research proposal combines molecular genetics with neurobiology and cell biology to define the role of the weaver gene in cerebellar development. Mouse microsatellite markers that map close to the gene will be used to identify embryonic weaver homozygotes. This technology will be used to examine the development of the cerebellum in weaver embryos prior to the apparent morphological manifestations of the mutant. In situ and immunocytochemistry analysis will be used to answer these questions. In addition, reaggregate cultures with embryonic weaver granule cells will determine the earliest time in granule cell development that the mutation has an effect. Rescue of these cultures with wild type granule cells will determine the time in cerebellar development that the weaver ligand is present.